In an effort to protect vehicle operators and occupants, vehicle manufacturers and their suppliers are increasingly designing and manufacturing vehicles with additional and improved safety features. One such safety feature that has been incorporated into a number of vehicle types is anti-lock braking, which can take a variety of forms.
Vehicle anti-lock brake systems (ABS) are designed to maximize the ability of a vehicle operator to bring a vehicle to a controlled stop on any type of road surface. The system accomplishes this goal by preventing the vehicle brakes from prematurely halting vehicle wheel rotation, or "locking" the vehicle wheels, regardless of the road surface and the pressure applied to the brake pedal by the vehicle operator.
Typical ABS comprise a vehicle wheel speed sensor as well as a brake switch for providing input to a control unit that controls ABS valves interposed between the master brake cylinder and the individual wheel brakes in the vehicle hydraulic brake circuit. The valves, in turn, regulate hydraulic brake fluid pressure in the individual wheel brakes to implement anti-lock braking.
In operation, the vehicle wheel speed sensor not only measures the vehicle wheel speed, but also provides input to the control unit for determining a vehicle speed. The brake switch indicates to the control unit the application of the brake pedal by the vehicle operator as a result of the operator's desire for vehicle braking. Typical ABS control units provide for anti-lock braking only in the event of an indication from the brake switch that the vehicle operator has called for vehicle braking through brake pedal application.
After application of the brake pedal by the vehicle operator, the control unit begins to monitor the vehicle wheel speeds for an indication of an anti-lock braking event. First, based upon an estimated vehicle speed, the control unit typically determines a slip threshold, which may be expressed as a selected velocity less than the vehicle speed.
Next, the control unit compares the vehicle wheel velocity to the slip threshold. During normal vehicle braking, the wheel velocity closely matches the vehicle velocity. However, during an ABS event, the wheel velocity decreases significantly below, or "departs" from, the vehicle velocity, and approaches the slip threshold.
In such a situation, as for example during hard braking on an ice covered road, the torque generated by the brake caliper and rotor assembly may exceed that generated by the vehicle wheel and the road surface. Uncontrolled, such a torque differential causes the vehicle wheel to cease rotating, or to "lock." In turn, locking causes the vehicle wheels to slip or "skid", rather than roll, over the road surface. Such vehicle wheel skidding dramatically reduces traction and the ability of the vehicle operator to bring the vehicle to a controlled stop.
To prevent such vehicle wheel lock and the accompanying problems, the ABS control unit activates the ABS valves to regulate brake fluid pressure in the individual wheel brakes during an ABS event. More specifically, the control unit compares the wheel velocity to the slip threshold and actuates the ABS valves when the wheel velocity falls below the slip threshold. In so doing, the individual vehicle wheel brakes in the hydraulic brake circuit are isolated from the master brake cylinder, thereby halting any increase in brake fluid pressure in the vehicle wheel brakes and preventing incipient wheel lock.
As previously mentioned, however, typical ABS provide for control of the vehicle brakes only where the control unit has received input from a brake switch indicating that the vehicle operator has applied the vehicle brake pedal in response to a desire for vehicle braking. As a result, in the event that the brake switch fails, ABS cannot and will not be activated, despite the fact that the vehicle operator may be applying the vehicle brakes and causing significant departures between the vehicle velocity and the wheel velocity. Thus, the entire purpose of ABS is corrupted by the failure of a single component.
It has therefore become increasingly desirable in the design of ABS to provide for fault tolerant operation. More specifically, it has become increasingly desirable to provide a control method and system for ABS having reduced brake switch dependence so as to provide for continued operability in the event of brake switch failure. Such an improved control method and system would also increase robust operation of ABS.